Silver-silver oxide catalyst on fused beryllia



Patented Aug. 5, 1952 UNITED STA ES PATENT f ,oFFIcE '7 i sILv R-s-i vER, OXIDE CATALYST N 7: V i ,FUSED BERYLLIA 'fBlld-Ollfll L Heide'r, Dayton, Ohio, assignor to Monsanto Chemical Company, St. Louis, Mo., a

' corporation of Delaware 7 No Drawing. Originalapplication Mama,

7 19.48, Serial No. 16,140. Divided and this ap- 1 v plicationNovember 18, 1950, Serial No. 198,741

This invention relates to the manufacture of ethylene oxide; and more particularly to the process of producing ethylene oxide by the catalytic oxidation of ethylene in "contact withthetombination catalyst of silver deposited upon berylliumoxide. p x i An object of the invention is the production of ethylene oxide by catalytic oxidation of technical gas mixtures, particularly-in processesin which temperature urges'may occur. Another object is the "provision of a catalyst-which obviates the necessity for longv and careful aging, or low temperature operation during reduction which has been characteristic of the catalysts of the prior art. Still another object of the invention is the provision of a silver bearing catalytic material which includesa carrier composed of beryllium oxide particles suitable forthe catalytic oxidation of ethylene to ethylene oxide.

Other objects will appear hereinafter.

' The catalytic oxidation of'ethylene for the production of ethyleneoxide has been widely'investigated and it has been found thatwhile silver or silver oxide is a catalyst for the reaction, the efficiency of this oxidation is determined largely by the nature of thecatalyst carrier upon which the silver is supported or deposited. Catalyst carriers employed in the'prior art include'pumice, carbon, silica, zeolite, kieselguhr, Alundum, corundum, etc. I Some of these carriers, for example, zeolite, give only very low conversions per pass. o

' Thesilica carriersarealso subject to various disadvantages; F or example; silver on silica gel tends to develop hot spots'during the reaction so that carriers employing'this-material are soon deactivated and :become worthless as a catalytic support. The silver may-also become catalytical- 1y 'inefiect'ive byreason' of a reaction, ith the silica to form silver "silicates. "Some forms of silica also exhibit a further disadvantage because they promote a combustion reaction'forming CO2, which mustbemitigated by means of surface blanketing materials or promoters, such as barium and'calcium peroxides and other, salts. These, however, are only'partly efiective' in reducing the activity of these silica carriers and it is 'found that the silica maintains its directive influenc towards complete combustion without the formation ofhighyields of ethylene oxide whichar desirable; j V i The alumina class of materials, such as carriers based upon Alundum or corundum, when employed incombin'ation with silver give good 0 1- versions to'ethylene oxide, but their initial high activity requires theus' of very low concentrations of ethyleneQin the gas mixture which is subjected to these catalysts. Thus,- while theconversions obtained with these alumina catalysts are good, the output of ethylene oxide is low. Another disadvantage of the alumina carriers is that the "silver salts on the surface of these materials must be reduced 'at a very low temperature, such as temperatures of approximately C. for the reductions which are necessary to maintain the activity of a catalyst.

The necessity for low temperaturereduction is described in U; S. Patent No. 2,142,948., Here the reduction of silver upon an aluminouscarrier was carried outfirst'at 250 C., and all subsequent reductions jwere'c'arried out at 150 0., whereas the ethylene oxide reaction-itself was carried out at 290 C; to 295 C.

It is recognized by those skilled in the art that lower reduction temperatures are necessary with the conventional alumina based carriers if an active silver catalyst is to be produced. In obtaining an active form, howeven-the resultant silver crystallites are very small and display poor wetting with resultantpoor adherence to the alumina carrier. The beryllia carrier, in contrast toalumina and silica carriers, appears to be more thoroughlywetted by the deposited silver film. For this reason the silver is held more tenaciously, so thatit is not readily flakedor s'loughed off by erosion of thegas stream in the operation of the ethylene oxide process.

It has been found-by microscopic examination of catalyst samples that the low temperature reduced silver formedu'p'on such aluminacarriers exists in a very'finely dividedst'ate. When such low temperature reduced silver-alumina catalysts are usedfor ethylene oxide production at conventional temperatures; it has been found that the micro-crystallitesof silver gradually increase in effective size during use,- and that the crystallites eventually reach a larg er size. This growth of the silver crystallite-is further accentuated when hot spotsdevelop during normal operation as by variations in processconditions; iAs a direct result of such crystargrowth the extensive at a relatively low temperature which is free from thermal fluctuation.

It has now been found that the combination of silver upon fused beryllium oxide provides an extremely rugged catalyst which obviates the prior 5 art requirements of low temperature reduction, and which can withstand high temperature surges.

The new catalyst may also be employed in an higher catalyst temperatures than was possible in the past. Since this catalyst is less subject to;

crystallite growth of the deposited silver, itis far safer as a commercial catalyst, since it can with-- stand temperature fluctuations. and local overheating such as may occur, for example; in the burning of carbon particles resulting, from the-. presence of traces of acetylene'in' the feed gases."

It has been conventional in the priorlart operations utilizing silica or alumina based carriers to employ inhibitors or repressants in ethylene oxide production. For example as shown in Patent No. 2,279,470 control ofthe. complete combustion to carbon dioxide'hasbeen attemptedby the addition of various compounds to mitigate such undesired reactions; However, the present silver-beryllium oxide catalyst, because, of greater heat-stability'isnot as prone to require inhibitors or repressants, since this catalyst is not subject to the deleterious effects caused by sudden temperature surges.

Because of the. temperature stability of the fused berylliurrr oxide carrier in combination with silver, it is possibleto employ not only pure ethylene for ethylene oxide production, but to 5 utilize commercial gas mixtures aswell, including leaner ethylene concentrations than were heretofore possible, for example, 90 or lower purity in the hydrocarbon feed stream. Thus, in addition to the possible contaminants, such as 40 acetylene, which may result from the use of byproduct ethylene obtained in acetylene. production by high temperature combustiomgasesobtained in petroleum cracking operations may be employed. Such still release gases, for example, 5 e H A H V H nated upon the carrier as metal or salt, the, cataas those containing ethylene other gaseous ole fins, and parafllns, "aswell as acetylenes maybe utilized for the production of ethylene o xide by the catalytic oxidation of the ethylene contained in such gas mixture by my present invention. While Lenher in U. S. Patent No. 1,995,991 suggests that he may oxidize arnixtureot hydrocarbons or a gas containingan olefin hydrocarbon (page 3,001. 2, lines 61 -63), theprpcess employed by him is non-catalytic and results in the production of more aldehyde-than ethylene oxide. I have found that whena mixture of air with paraflinic hydrocarbons containing ethylene is passed over conventional silica or alumina based catalysts which were known to give conversions of pure ethylene to ethylene oxide, there occurs so rapid an oxidation of theparaifinic constituents otjthe mixture that overheating of the catalysts, resulting in deterioration of the same, always takesplace. The use of gaseous 65 hydrocarbon mixtures containing parailinic hydrocarbons, as well as the acetylenic hydrocar bons such as may occur in high temperature hydrocarbon processing, is, of course, preferable from an economical standpoint to the use of pure ethylene. The new catalyst and the process employing the same herein fprovided obviates the necessity for purifying the-ethylene previous to. the oxidation step so that the objective of the production of ethylene xide from such cheap 4 and readily available gas mixtures may be realized.

The beryllium oxide may be employed in any desired form or condition. Granules of the oxide are convenient for disposal in catalytic converters. A preferred embodiment is the use of beryllium oxide in fused form. This state of the oxide is very stable and resistant against degradation by the gas stream which is passed through the ethylene oxide process operating at consi derfably iecatalylstbed at a high velocity during the catalytic reaction. The fused beryllium oxide may be utilized in the form of fine particles or as a relatively coarse aggregate, dependent upon the particular. types of reactors which may be used and upon the allowable pressure drop. It is an advantagev of the beryllium oxide that the ruggedness of this material prevents the breakdown or powdering of the particles so that the initial particle size is substantially maintained throughout the life of the material in the catalytic converter. Consequently, the pressure drop does not change greatly duringithe life of the catalylst, since the initial particles are themselves maintained intact .without degradation, and the deposited silver is also; held with extreme tenacity without the production of silver globules upon the surface. The beryllium oxide. is preferably employed as the solecarrier of. the. silver catalytic material, but mixtures of beryllium oxide with other carriers may be employed. However, the percentage. of beryllium oxide as an essential constituent in the carrier must. be maintained sufiiciently high so thatthe characteristic qualities such as high temperature stability, adherence ofsilver and high initial ethylene feed concentration remain substantially unaffected.

The combination of the; fused beryllium oxide with silver. may be accomplished by impregnating or admixing withanyconvenient silver salt. The beryllium oxide. particles may be slurried with the silver salt or maybe impregnated therein. Alternatively .a slurry of finely divided metallic silver or silver oxide particles may also be'applied tothe beryllium oxide carrier.

After thesilverhas been deposited or impreglytic. material is dried andmay be reduced before charging to, thereactor, or may be, reduced in situ inthetreactor. a

The reduced silver ,material',. which. in association with the" beryllium oxide carrier is the active constituent in the'ethylene oxide process. is. probably metallic silver. but may be silver oxide, and. may. also include variable. proportions of silver oxide; 1 The. totalsilver content. may vary from 5%. to 25%, .reiierred .to, the. carrier.

The reduction process Lllemploying; beryllium oxide as a base be carriedout at farhigher temperatures than has been. possibleLwith prior, art materials; Thus, silvernitrateimpregnated upon berylliumv oxide may belreduced at. about 275 C. to 3001? Cawithout, the.dange r. of fusion of the silveron the catalyst surface, With silver nitrate deposited. upon alumina in the .samev manner, itis. found that the. temperature must be held; about 10.03 C. Iower'than. inthe, case. of beryllium oxide. Y i I I] j f The catalytic operation employing the. silver-- beryllium.ioxide.- catalysteitherv in. the presence. or absence. of other gaseous hydrocarbons. may be carried out at temperatures o,;from.22,5. C., to 350 C. and preferably atiemperatures of aboutfl250 C. mean: C.' A .preferred emhodiment is the operationof 'ethylene.. oxidation, utilizing beryllium. oxide. at; about; 2'15? C. The...

oxidizing medium may be air or a mixtureof oxygen'and' anjinertgasssiich as may result from oxygenen'richment, Oxidation may, be economically ;carried" out, by a single pass ioperation al though,' ifi desired, recycling r," the exit. gases (from which the ethylene "oxidehas, been removedjby, absorption) may be used by constant additionfof ethylene" and ,airto the feed, gases. By em'ployingthe presentcatalyst, from 70% to 80% .01v the ethylene originallylpresent inlthe feed gas isconsumedjinfone"pass, irom 40%}to 45%l-an'd higher .being c'onverted to. ethylene oxide and the remainder to other. oxidation products, chiefiy carbon dioxide. Thus, after absorption of "the ethylene oxide-the resulting gas mixture generally contains less than 1% of ethylene, the major proportion of'this gas stream being carbon dioxide, oxygen andnitrogen. In a specific instance, for example,.,employing an initial gas mixture consistingof 3% by volume of ethylene andi97 %.by;volume' of air, 45% of the ethylene is converted into ethylene oxide and30% into carbon dioxide.

In orderv tov present the invention in greater.

detailyseveral examples are set out below; although it is intendedjthat the subject matter of the said examples shall be illustrative and not limitative offthe present invention.

. f Example 1 Fusedberyllium oxide in the form of: 4- x 8 mesh particles was dried in a vessel immersed in a nitrate'bath at 300 C. for'one hour. During this time-the vessel was evacuated by applying full water pump suction. After cooling the beryllium oxide, a -twater solution containing 25% by weight of silver nitrate was added to the carrier in such amount as'to produce acarriersilver nitrate containing,2 5% byweightof the latter salt based ontheweightof the carrier. The mixture was allowed to soak overnight. The excess water was removed under vacuum by heating the carrier in the same vessel. The dried silver nitrateimpregnated catalyst was then charged to a heated reactor and reduced with hydrogen at 275 C. to 300 C. for 5 hours.

Example 2 The utilizationof the catalystof Example 1 i an ethylene oxide process was carried out by charging the impregnated catalyst to the same stainless'steeltube in which the reduction was carried out. The tube was surrounded with a constant temperature bath.

The reduced catalyst was operated in a recycle system. The make-up gas mixture of 7% ethylene and 93% air, was mixed with 4 parts of the recycle gas from which the ethylene oxide had previously been absorbed. The total mixture was then passed over the catalyst at a space velocity 1300 R. H. t

It was found that space velocities could be utilized at a far higher range than was possible with prior art catalysts which generally operated at about 500-700 reciprocal hours (R. 1-1.). By the term space velocity expressed as reciprocal hours (R. H.) I mean the number of volwires of gas measured at standard conditions (0 C./760 mm.) per hour passing over a unit volume of catalyst. The thermal stability of the beryllium oxide in avoiding hot spots, together with its quality of holding the deposited silver, apparentlyallowed the use of far higher velocity gas streams without permitting the hot spots to reach the dangerous condition of a runaway reaction which might propagate a; flame and fuse the'catalystq .fhave lfound; jmfo'reQver; thatijaside from" the economic "consideration of high j,throughput the space velo'cityj eanflbe considerably varied, de-

pending" upon btheriactors"entering into the operation, i. je., {reaction temperature concentration of ethylene an 1 1a .irv in the! gas mixture proportion of beryllia used in the. catalyst, etc. Generally thejspace velocity required for good conversions -to ethylene woxidefv'aries from 500 to 3,000 R. H. While ordinarily a space velocity in the range of 1,000to 1.500 R. H. will be employed, thisvalue maybe varied,"particularly if a recyclesystem isicontemplated.

The operations in the present process were carried out utilizing a recycle ratio of 4:1. However, this recycle-ratio may be varied considerably," depending upon the type'jfof product which is. desired, since it is possible when producing ethylene glycol directly from the exit gas stream toloperate at considerably higher'recycle ratios such.as.8:'l. {,j

- In the production of ethylene oxideas such, I prefer to employ low recycle ratios such as 2 1, and may also use a nce through process;

. M e-' A comparison was made ofithe present beryllium oxide in combination with silver as a catalyst relativeitoqthepresults-;obtained in anotherexperiment made with the use of asilver onjsilica. This catalyst consisted of 23% silver deposited on 4 x 8 mesh silica particles. This catalyst wasiemployed with a gas mixture consisting :of 3% of ethylene and 97% of air by volume. The-temperature of the bath surrounding; the catalyst; tube was maintained at about 244 C., which was. as high as the temperature could. safely beabrou'ght' for continuous operation. During the -i irst 2.5 days the temperature of the catalyst, bed rose-from 266 C. to, 349 C. and the per centiconversion of-intro-- duced ethylenereacting to form ethylene oxide fell from 38. 1%..to 26.7%. Y The ethylene andair fiows;were"2'.0 and 65.0 liters per hour, respec-. tively: corresponding to a space velocity of i in another test which; was sae to compare 1100 R. H. and the catalyst bed temperatures of from 310 C. to 364 C. varied from 8.8% to 13.9%.

In the operation of the present process it was found that no preliminary aging of the catalyst is necessary, and that the operating temperature may be increased to a considerably higher temperature than was previously possible by reason of the greater thermal stability of the catalyst. This thermal stability is attributable to the nature of the combination of silver with beryllium oxide, since the present catalyst combination is considerably less susceptible to surface fusion of the catalytic silver present on the beryllia. The present catalyst is consequently also uninfluenced by accidental temperature increases which are inevitable in any technical process of 7 7 this nature; I have found that such accidental temperature increases which might raise the catalyst temperature, to values exceeding 350 C. do not influence the. conversion. although such occurrences areknownjto render an ordinary alumina catalyst unflt'for further use.

The present process'utilizing the catalyst of beryllium oxide in "combination with silver was founclunder the above conditions 'to give conversionsfof.40%,to, 45% to ethylene oxide. and yieldsof 55% to 50%jbased upon the ethylene consumed.

The above reaction was carried out at atmospheric pressure, although subsequent work has shown that the reaction proceeds equally well at pressures above or below atmospheric.

Recovery of the ethylene oxide may be eflected by any ,of the methods known to the art. i.'e'.. by absorption upon activated carbon or by solution in water or other solvents. Preferably, the reaction gases are passed through a scrubbing tower wherein the ethylene oxide-containing gases are thoroughly contacted with water or other solvents, and the ethylene oxide dissolved therein for further processing.

The present combination catalyst of silver deposited upon beryllium oxide is particularly advantageous in large scale processes for the production of ethylene oxide.- In a commercial unit it is highly advantageous to employ the same converter for the'reduction of the catalyst and for the oxidation step, partlcularly'if both operations are performed at the'same converter temperature. j I

' The requirement of-'-reducing alumina and silica supported silver catalyst at low temperatures in order to obtain the desired activity necessitates the addedcapital expenditure of another converter for'the sole purpose of preparing catalyst. The present-silver-beryllium oxide catalyst avoids these difiiculties since it may be reduced at the normal operating temperature without losing its activity.

It has been observed that theberyllia-silver combination catalyst'permits the use of higher concentrations of. ethylene such as ethylene in air in the incoming gas. It has been a characteristic of the prior art alumina-silver-catalyst that the ethylene concentration had to be kept at a low value, particularly during the initial break-in operations, an only slowly raised after many hours of operatingtime until the optimum ethylene concentration could be tolerated.

T In'contrast'to this situation. the present beryllia-silver catalysts allow the'higher ethylene feed gases to be used immediately after thereduction of the catalyst has been completed. Because of this gain in [the ability to utilize high ethylene concentrations the present catalysts permit a. process to be carried out in which gradual and constantly changing addition of ethylene need not becontrolled manually.

,Since many widely different embodiments of this, inventionmay be made without departing from the spirit thereof, it is to be understood that I do not limit myself to the specific embodiments thereof except as defined in the appended claims.

The. present case is a division of Serial No. 16,140, flled'March 20, 1948,, now U. S. Patent No. 2,554,459. 7

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A silver catalytic material of the group consisting of silver oxide, silver, and mixtures thereof suitable for the catalytic oxidation of ethylene to ethylene oxide which is deposited upon a. carrier composed of fused beryllium oxide particles associated with the said silver catalytic material. the silver catalytic material composing from 5% v to 25% by weight of said carrier.

2. A silver catalytic material consisting of silver deposited upon a carrier composed of fused beryllium oxide particles associated with from 5% to 25% by weight of the said silver catalytic material.

3. A silver catalytic material consisting of silver oxide deposited upon a carrier composed of fused beryllium oxide particles associated with from 5% to 25% by weight of the said silver catalytic material.

4. A silver catalyticmaterial consisting of a mixture of silver and silver oxide which mixture is deposited upon a carrier composed of fused beryllium oxide particles associated with from 5% to 25% by weight ofthe said silver catalytic materiah RUDOLPH L. HEIDER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PA'I'ENTS 

